We propose major improvements in the design, construction, and operation of in vivo electron paramagnetic resonance (EPR) small animal imagers and thereby facilitate quantitative and qualitative enhancements in the translation of EPR to a wide range of physiologic problems, with a focus on tumor imaging. Treatment of cancer requires knowledge of the O2 tension in the tumor, and development of new drugs requires molecular level understanding of tumor physiology. EPR imaging can noninvasively obtain an O2, pH, and redox image of the tumor and surrounding tissue, and correlate images with treatment outcome. No other available in vivo O2 images provide such a combination of accuracy of the O2 tension and lack of confounding biologic variability together with low level of invasiveness and low toxicity. Accurate O2 imaging could be the basis for precise dose painting in radiation oncology. This project is a partnership between the University of Denver (DU) and Bruker BioSpin. Bruker is the largest supplier of EPR instrumentation, worldwide, with expertise in both academic and industrial settings and currently sells a continuous-wave-only L-band small animal EPR imager. Bruker and DU will develop a new imager implementing innovations by Bruker and the DU EPR Center, including low- frequency pulsed EPR, digital EPR at multiple frequencies, rapid-scan EPR methodology, crossed-loop pulse and rapid-scan resonators, rapid-scan driver systems, air-core magnets, current control of magnetic fields during imaging, fast-response RF power amplifiers, low-power pulsed EPR, and new operator-friendly tuning displays. The new pre-commercial prototype in vivo imager will provide capability to measure tumor O2 levels, which are needed for oncology treatment, and will provide capability to measure tumor pH and redox status in addition to O2 tension to understand the physiology of tumors in support of pharmacological developments. The magnetic field will be current-controlled, so the problems with Hall probe location in magnets that use Hall probe feedback control of the field will not occur. The imager will be a very flexible system based on digital EPR concepts in which an arbitrary waveform generator (AWG) and fast digitizer replace much of the current hardware. The prototype will be optimized to exceed current art and be faster, better, and less expensive than existing in vivo EPR imagers, and with user-friendly software to make it easy to operate by people who are not spectroscopists. We will document readiness for manufacturability and commercialization. The new technology developments and sharing of information will result in a prototype small imager that Bruker will then further develop with corporate funds to sell commercially.